The field of fiber optics is currently undergoing rapid growth. A good portion of this rapid growth is driven by the desire to provide larger bandwidth capability to communication systems. Many approaches have been utilized to increase the existing communication system's current infrastructure. Improvements in amplifiers, encoding and decoding techniques, as well as others, has had some success. Of technologies currently available, optical fibers hold great promise for increasing available bandwidth. Thus, optical components that are able to assist in attaining increased bandwidth are of interest.
One of the optical components that is of value is an optical isolator. By providing the ability to transfer light in essentially one direction only, the optical isolator may "shield" those components whose performance suffers from light traveling in a reverse direction. Devices such as optical amplifiers, lasers, and other components suffer performance degradation from light entering in a "reverse" direction.
One prior optical isolator provides single fiber optical isolation. However, such an approach also introduces polarization mode dispersion because of the differing optical paths. These differences in optical path length may be due to such factors as differences in refractive indices, and differences in physical paths. For example, FIG. 1 illustrates an optical isolator 100, in which differing optical paths for different polarizations introduces polarization mode dispersion. In this prior art, incoming light and exiting light are coaxial. Incoming light 104 has random polarization as illustrated by horizontal and vertical components 102. The incoming light 104 upon entering a birefringent crystal 150 travels different paths depending upon polarization. Path 106 is the path for the horizontally polarized component of the incoming light 104, while path 105 is the path for the vertically polarized component of the incoming light 104. The horizontally polarized component of the incoming light 104 upon exiting the birefringent crystal 150 continues 108 traveling through 110 the Faraday rotator 160, then 112 through 114 a half wave plate 170, then 116 through 118 a second birefringent crystal 180, and exiting 120. The vertically polarized component of incoming light 104 upon exiting the birefringent crystal 150 continues 107 traveling through 109 the Faraday rotator 160, then 111 through 113 a half wave plate 170, then 115 through 117 a second birefringent crystal 180, and exiting 120.
The optical path length for the horizontally polarized component of the incoming light 104, in this example, is shorter than the optical path length for the vertically polarized component of the incoming light 104. It is this difference that contributes to the polarization mode dispersion. That is, incoming light 104 may have vertical and horizontal polarization components that are coincident with respect to each other upon entering the isolator 100, however upon exiting the isolator 100, the horizontally polarized component of the incoming light 104, in this example, travels a shorter distance than the vertically polarized component of the incoming light 104 and so the horizontally polarized component of the incoming light 104 will exit the optical isolator 100 before the vertically polarized component of the incoming light 104 with which it was initially coincident. Because of this spreading, signals must be spaced further apart in order to resolve them. This results in less than optimum information capacity.
Another prior single fiber optical isolator solution concentrates on lowering polarization mode dispersion. Yet, other optical isolators use components that try to address the assembly of thermally expanded core fibers via the use of V-groove techniques. FIG. 2 illustrates a V-groove assembly 200 in which individual fibers 204, 206, 208, and 210 are positioned within V-shaped grooves 214, 216, 218 and 220 respectively that are fabricated on a substrate 202. V-groove assemblies require steps to place and secure the individual fibers within the V-grooves. Additionally the pitch between the fibers is currently in the 250 .mu.m range. The alignment of an input V-groove assembly with other optical components and then with an output V-groove assembly presents challenges.
The prior art discloses single fiber optical isolators that may suffer from the introduction of polarization mode dispersion and multiple core assemblies that may suffer from assembly difficulties.